Numerous metathesis catalysts are already known; refer to for example: Tetrahedron Lett. 1999, 40, 1091-1094, J. Am. Chem. Soc. 2000, 122, 58-71, Angew. Chem. 2003, 115, 1944-1968. It is known from these publications that substrates comprising acetal-groups considerably inhibit metathesis reactions.